This invention relates to rotary mechanisms of the type having housing chambers with at least one lobe therein and wherein each lobe has a sharp transition point, and which may be employed as engines, compressors, pumps, expanders, or the like.
Prior art of possible relevance includes U.S. Pat. No. 3,323,498, issued June 6, 1967 to Kraic et al.
Rotary mechanisms having rotors within a chamber which undergo both rotational and translational motion within the chamber have lobed housings and can be characterized, in terms of their housings, as being in two categories. The first category, and the one with which the present invention concerns itself, is that type of rotary mechanism wherein the lobe or lobes within the chamber have relatively sharp transition points. The second type is comprised of those rotary mechanisms wherein the lobes are continuous curves and, consequently, do not have sharp transition points. An example, of the former type is a hypotrochoidal mechanism, while examples of the latter type include epitrochoidal mechanisms and slant axis rotary mechanisms.
In rotary mechanisms having epitrochoidal housings, typified by the "Wankel" engine, long life has been a long-sought goal. The principal difficulty in achieving the goal has been the inability to provide long lived seals, particularly apex seals in the rotor. In designing such mechanisms, the housing geometry is determinative of rotor shape with the result that the rotors have rather sharp apexes and very narrow apex seals. As a consequence, according to the present state of the art, in a typical epitrochoidal engine, sealing contact between an apex seal and the housing shifts no more than about 0.05 inches on the surface of the apex seal. The same may be said for slant axis rotary mechanisms typified by the "Clarke" engine. The confinement of sealing contact to such a small area causes rapid wear, even when exotic materials are employed in forming the seals.
Kraic et al., in their above-identified patent, propose a rotary mechanism superficially similar to epitrochoidal mechanisms, but falling in the first category of housing shapes as enumerated above. According to Kraic et al., the rotor of the mechanism is formed as a hypotrochoid and the housing geometry is that of the clearance envelope of the hypotrochoidal rotor. That is, the housing geometry is generated by the loci of points on a nose of the hypotrochoidal rotor when the same is rotated and translated in the desired fashion such that the noses of the rotor would always be in contact with the housing. In such a hypotrochoidal mechanism, the noses, corresponding to the apexes in epitrochoidal devices, or slant axis rotary mechanisms, are quite rounded and contact the housing at various points of operation over a considerably greater area than in either epitrochoidal or slant axis rotary mechanisms. As a consequence, seal wear is not confined to a small are and therefore, seal life is greatly prolonged.
However, such devices require lobed housing walls wherein the lobes have transition points that are relatively sharp, i.e., have very small radii. Thus, while following the approach will provide long lived seals, the life of the housing is shortened by reason of high contact stresses imposed upon the lobes by seals carried by the rotor. Such high contact stresses shorten the life of the housing. Specifically, because of the high contact stresses, the lubricating oil film is virtually non-existant. As a result, friction is exceedingly high and the temperature of the rubbing surface at the point of contact (the conjunction temperature) is also quite high. This causes scuffing and rapid wear resulting in housing failure.
The same difficulties attend rotary mechanisms in the first category enumerated above other than hypotrochoidal mechanisms. Consequently, the advantage of long seal life is at least in part lost due to the shortening of the life of the housing.